Extending excited-state lifetimes by interchromophoric triplet-state equilibration in a pyrene-Ru(II)diimine dyad system

The synthesis and the spectroscopic properties of a bichromophoric ruthenium trisbipyridyl-1,4-diethynylenebenzene-pyrene system (Ru-b-Py) and the corresponding pyrene ligand (b-Py) are reported. The ruthenium model systems Ru-b-OH, Ru-b-Ph are also presented. UV-Vis absorption and emission at room and low temperature and time-resolved spectroscopy are discussed. For the Ru-b-Py dyad, a mixing of the (MLCT)-M-3 state of the ruthenium-based component and the triplet state of pyrene, Py-3, is observed. Time-resolved transient absorption studies performed on the Ru-b-Py and on the b-Py ligand show that the lowest energy absorption is due to the population of the triplet state localized on the pyrene-component. Time-resolved studies also evidenced a relatively slow forward triplet equilibration rate, in the order of 2 x 10(5) s(-1) (5 mus), and an even slower back energy transfer rate, 3.3 x 10(4) s(-1), still faster than the intrinsic decay time of the pyrene (200 mus).